gistfile1.js

/* 
  evaluates as a block — "{" and "}" — containing one labeled statement — `a:"a"`.
  labeled statement, in its turn, evaluates to "a".
*/

{a:"a"}

/*
  evaluates as an expression, where "(" and ")" are grouping operator and "{" and "}" constitute an object literal.
  evaluates to this object value.
*/

({a:"a"})

/*
  syntax error, `{a:"a",b:"b"}` is evaluated as a statement and statement can not have such syntax.
  enforcing expression context — `({a:"a", b:"b"})` — fixes this "problem".
*/

{a:"a",b:"b"}